The invention relates to a gas exchange valve mechanism for an internal combustion engine. In particular the invention relates to a gas exchange valve mechanism for an internal combustion engine operable to selectively open and close a gas exchange valve to accomplish an engine brake during an engine brake mode of an engine.
In order to provide efficient braking systems on vehicles, supplementary brake systems to the disc or drum brakes are frequently provided. An example of such a system is an engine compression braking system. Engine compression braking generally operates such that the engine is operated in a braking mode where little or no fuel is supplied to the cylinder or cylinders operating in a braking mode. The piston is allowed to compress the gas present in the cylinder. Before expansion takes place a valve is opened to discharge the compressed air. Power is consumed during the compression of the gas which results in braking of the engine. The control of opening and closing of a valve included in the engine compression braking system may be performed by valve mechanism for an internal combustion engine operable to selectively open and close a valve to accomplish an engine brake during an engine brake mode of an engine. The gas exchange valve mechanism includes a cam follower in biased abutment against a cam element for actuation of a rocker arm connected to said gas exchange valve. The rocker arm is connected to the valve for controlled actuation thereof. The compression and discharge of compressed air may suitable take place during a compression stroke of the engine. An example of an engine compression braking system is provided in U.S. Pat. No. 6,253,730.
U.S. Pat. No. 6,253,730 relates to an exhaust valve mechanism for an internal combustion engine. The exhaust valve mechanism includes a cam shaft having an exhaust valve cam element including a main lift portion and at least one brake lift portion. During the main lift portion the rocker arm opens the exhaust valve or valves of a cylinder to reach a fully open position to enable scavenging of exhausts during a normal power mode of the engine. During the brake lift portion the gas contained in the cylinder should be discharged after having been compressed at a compression stroke in order to achieve engine braking. The brake lift portion is considerably smaller than the main lift portion and occurs preferably at the end of the compression stroke. A further brake lift portion may be provided at the beginning of the compression stroke or just before the compression stroke in order to admit gas at higher pressure at an exhaust manifold enter the cylinder when the gases in the cylinder has a lower pressure which is the case around bottom dead centre of the piston.
The exhaust valve mechanism according to U.S. Pat. No. 6,253,730 furthermore includes a rocker arm mounted on a rocker arm shaft, said rocker arm provided with a cam follower in biased abutment against the exhaust valve cam element for actuation of the rocker arm.
In order to allow the gas exchange valve mechanism to operate during both normal power mode of the engine and engine compression braking mode, the gas exchange valve mechanism further includes an actuator piston bore formed in the rocker arm. The actuator piston bore defining an actuator cylinder housing an actuator piston slidably arranged in said actuator cylinder. The actuator piston is arranged in abutment with a gas exchange valve for selective actuation of the gas exchange valve. A braking fluid circuit is formed in the rocker arm. The braking fluid circuit connecting said piston bore with a control valve selectively connecting the braking fluid circuit to a braking fluid supply line or a braking fluid drain line. Depending on whether the braking fluid circuit is connected to the supply line or drain line, a high or a low brake fluid pressure will be present on the remote side to the actuator piston with relation to reset valve, the During normal operation the actuator cylinder will not be supplied with high pressure braking fluid and the actuator piston will not transmit sufficient pressure to the gas exchange valve to obtain opening of the same. Instead, the actuator piston will be pressed into the actuator cylinder. When the cam follower enters the main lift portion a contact shoe is engaged with the gas exchange valve or a gas exchange valve overhead to allow the continued movement of the rocker arm to open the gas exchange valve during the gas exchange gas discharge stroke.
The gas exchange valve mechanism further includes a reset valve being arranged in the braking fluid circuit to open or close a connection between said piston bore and said control valve. Opening of the reset valve allows discharge of brake fluid from the actuator cylinder and closing of the gas exchange gas valve. A reset valve actuator mechanism controlling actuation of said reset valve is included to in the gas exchange valve mechanism. The reset valve actuator mechanism disclosed in U.S. Pat. No. 6,253,730 includes a reset contact element mounted on an engine component. The position of the reset contact element is such that a stem on the reset valve engages with the reset contact element when the rocker arm is at a desired location. That is the rocker arm must have lifted the exhaust gas valve to a certain position before unloading of the pressure in the actuator cylinder may take place.
This leads to that the opening of the reset valve cannot be performed independently of the position of the rocker arm. Since the opening of the reset valve requires that the rocker arm proceeds beyond the fully open stage of the gas exchange valve obtained by the brake lift portion, otherwise contact would already have been established between the reset contact element and the reset valve, opening of the reset valve is performed when the brake fluid in the actuator cylinder is set under pressure due to the compression of a valve spring made by the actuator piston. The necessity to open the reset valve when brake fluid in the actuator cylinder defined in the piston bore is set under pressure leads to increased wear of the components and increased requirements for structural rigidity of the components, which may lead to heavier and more bulky design of the arrangement.
It is desirable to enable slimmer design of a gas exchange valve mechanism for an internal combustion engine. It is also desirable to provide an arrangement which enables opening of a reset valve at a desired operating point independently of the pivotal position of the rocker arm.
The invention relates, according to an aspect thereof, to a valve mechanism for an internal combustion engine operable to selectively open and close a gas exchange valve to accomplish an engine brake during an engine brake mode of an engine. The valve mechanism includes a cam follower in biased abutment against a cam element for actuation of a rocker arm connected to the gas exchange valve.
The connection between the rocker arm and the gas exchange valve may be performed via an actuator piston housed in a bore formed in the rocker arm.
The actuator piston may be arranged in abutment with a gas exchange valve for selective actuation of the gas exchange valve.
According to the invention the valve mechanism further includes a reset valve actuator with a second cam follower in biased abutment against a reset valve cam element for actuation of a reset valve.
Since the actuation of the reset valve is controlled by a cam element the opening and closing of the reset valve may be accomplished independently of the position of the rocker arm. This enables opening of the reset valve in a situation when the gas exchange valve is closed.
The gas exchange valve may preferably be an exhaust valve.
The gas exchange cam element may be arranged on a cam shaft and include a main lift portion and at least one brake lift portion. The main lift portion may control the lift of an exhaust valve for scavenging of exhaust gases from a combustion chamber of the engine during an exhaust stroke under normal operation of the engine. The brake lift portion ensures that compressed gas may be released via the gas exchange valve during engine brake.
The valve mechanism may further include an actuator piston bore formed in the rocker arm. The actuator piston bore housing an actuator piston slidably arranged in said actuator piston bore. The actuator piston is arranged in abutment with a gas exchange valve for selective actuation of the gas exchange valve.
The valve mechanism may further include a braking fluid circuit formed in said rocker arm, said braking fluid circuit connecting said piston bore with a control valve selectively connecting the braking fluid circuit to a braking fluid supply line or a braking fluid drain line. The control valve selectively enables the valve mechanism to operate in either of a normal operation power mode or in a engine brake mode depending on whether the control valve supplies pressurized braking fluid to the piston bore or not.
The reset valve is arranged in the braking fluid circuit to allow draining of oil trapped in the open or close a connection between said piston bore and said control valve.
The reset valve cam element and said gas exchange valve cam element may be provided on a common cam shaft.
The reset valve cam element and said gas exchange valve cam element may be separate elements or formed as a single cam element. In any case two separate cam followers are used, one enabling control of the gas exchange valve via the pivotal position of the rocker arm and one enabling control of the reset valve element via a reset valve actuator. The use of two separate cam followers enables independent control of the reset valve in relation to the pivotal position of the rocker arm. It is hence possible to open the reset valve in an unloaded state when the rocker arm does urge the gas exchange valve into an open position. In this state the brake liquid in an actuator piston bore will not be set under pressure due to the contact between the rocker arm and the actuator piston, leading to lower wear of the components and lower requirements for structural rigidity of the components.
The second cam follower may include an arced concave portion faced toward said cam element. The use of an arced cam follower instead of a cam follower providing a distinct single point contact, such as the contact between a cam and a cam roller, enables extended contact between a cam peak and the cam during rotation of the cam shaft. A prolonged opening time of the reset valve can then be accomplished facilitating use of a common cam element for the gas exchange valve and the reset valve.
The reset valve actuator may include a reset member which is pivotally supported around an axis which is parallel and off set in relation to a pivot axle for said rocker arm. The reset member may be arranged to be pivoted by said reset valve cam element via the second cam follower. The second cam follower may be an integral part of the reset member.
The reset member may include a trace or opening having an extension in a circumferential direction allowing a reset valve stem to remain in an extended position while said second cam follower is positioned on a base circle of said reset valve cam element during movement of said first cam follower around the periphery of said exhaust valve cam element.
The reset member may further include a ramp portion extending in a radial direction from said trace or opening allowing a reset valve stem to transfer between an extended position when said valve stem is located in said trace or opening to a retracted position due to a relative movement of said valve stem and ramp portion in a radial direction following a pivoting movement of said reset member.
The reset valve stem may reach its fully retracted position at a maximum height of the reset valve cam element.